This invention relates to an improved method of making a luminescent screen structure for a cathode-ray tube and particularly, to a method of controlling exposure time in a screening lighthouse to account for variations in apertured color selection electrodes.
The direct photographic method for preparing a matrix or a phosphor screen usually includes coating the inner surface of the faceplate of a cathode-ray tube with a film of photosensitive material composition (typically comprising a dichromate-sensitized polyvinyl alcohol). A mask assembly is positioned in the faceplate and the entire panel assembly placed on a lighthouse. Multiple light beams projected through an aperture mask expose multiple areas corresponding to the apertures in the mask on selected regions of the film. After exposure, the panel assembly is removed from the lighthouse, and the film is flushed with water to remove the still soluble regions while retaining the insoluble regions in place.
Ideally, the exposed patterns formed on the film by each of the multiple light beams are of a desired size and shape. In practice, the ideal is not achieved, resulting in an exposed pattern of a different size and shape from each beam. This deviation from ideal is at least partly caused by variations in mask transmission. These variations in mask transmission may produce variations in the size of the exposed areas of the film. Generally, the greater the mask transmission, the larger the size of the exposed area.
When the mask light transmission is measured during the exposure, additional variations may be introduced by transients and nonequilibrium conditions in the measuring system. Also, variables may be introduced when the exposure cycle is not duplicated in successive exposures.
Such problems were partially overcome by the disclosure of U.S. Pat. No. 3,636,836 issued to W. J. Maddox et al. on Jan. 25, 1972. In this patent, the intensity level of the light transmitted through the faceplate is sensed with a photosensor and the light source is continuously adjusted to produce a desired luminence on the film. A disadvantage of such a method is that the sensed light must pass through layers of film and perhaps phosphor particles before it reaches the photosensor. Variations in film and/or phosphor thickness can affect the intensity of the sensed light. Therefore, there is a need for a light exposure control method that will not be limited by the foregoing disadvantages.